LONDON (Reuters) - Scientists on Wednesday said they have
an explanation how one of Saturn's moons can spew out a giant
plume of water vapor, adding to evidence a source of life --
water -- lies beneath the moon's frozen surface.

Using a computer model, German researchers showed the
temperature at the bottom of surface cracks on Enceladus has to
be about 0 degrees Celsius, the so-called triple point of water
where vapor, ice and liquid water all can coexist.

"This makes this moon very interesting for further study
because there is a connection between liquid water and life,"
Sascha Kempf, a physicist at the Max Planck Institut in
Heidelberg, said in a telephone interview.

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"This is the kind of thing planetary scientists hope for."

The scientists published their findings in the journal
Nature.

Mars, Jupiter's moon Europa and Enceladus are the only
places in the solar system with direct evidence of water.
Finding organisms different from those on Earth may provide
scientists with answers to questions ranging from where
diseases come from to how our sun and planets formed more than
4.5 billion years ago.

Scientists have taken an especially close look at Enceladus
because it seems to have a smooth surface -- suggesting recent
geological activity that, in turn, could mean liquid water.

They are also intrigued by the plume itself, a gigantic
geyser of water vapor and tiny ice particles. One mystery was
how the dust particles slowed down to keep the plume restrained
by the gravity of the moon, said Kempf, who worked on the
study.

Their model showed most of the dust particles collide with
the walls of the surface crack as they are ejected and
constrain the gas flow to keep the plume close to the surface
rather than shooting into the atmosphere, Kempf said.

The team used images of the plume and properties of the
escaping gas and dust particles to run their model. They found
it only reproduced the plume when the temperature was at zero
degrees at the bottom of the cracks, implying water exists
there in liquid form.

"The density of the gas jet inside the cracks is so high
that the small dust particles should have the same speeds to
escape from the gravity of the moon," he said. "If this was
true you wouldn't see plumes. You would see a long jets
expanding into the system."

Saturn has at least 47 moons and at least seven rings. The
joint U.S.-European Space Agency Cassini mission, launched in
1997, is spending four years examining Saturn.

Cassini is scheduled to fly 50 kilometers (31 miles) over
the moon's surface in March, which will provide more
information on the precise chemical composition of the
particles and water vapor as scientists try to better
understand the plume, Kempf said.